A series-connected, skin-current heating pipe herein defines a system including a pipe made from a ferromagnetic material (steel pipe, for example) and a wire which extends along and inside the pipe wall and is generally insulated from the pipe wall, with one end of the pipe and wire connected to two terminals of a single-phase a.c. source respectively, and with the other ends thereof connected each other. Such a skin-current heating pipe is disclosed in U.S. Pat. No. 3,293,407.
FIG. 1 shows a fundamental arrangement of a series-connected, skin-current heating pipe which has been widely known in the art. Reference numeral 1 designates a ferromagnetic steel pipe, 2 denotes an insulated wire extending along and inside pipe 1, and 3 refers to an a.c. source. Wire 2 and steel pipe 1 are connected in series with respect to power source 3. The skin depth S(cm) of the alternating current is expressed by: EQU S=5030.sqroot.p/.mu.f (1)
where .mu. is the specific magnetic permeability of steel pipe 1, p(.phi.cm) is the resistivity, and f is the a.c. source frequency. If the wall thickness t(cm) of pipe 1 is selected to be: EQU t&gt;2S (2)
it is considered that the alternating current flows along the inner periphery of pipe 1 within the substantially uniform depth S from the inner surface thereof and never leaks outside the steel pipe, thus providing a reliable heating pipe system.
The alternating current should be uniform throughout wire 2. However, in the event that a dielectric breakdown (short circuit) occurs at position 6, for example, in insulated wire 2 of FIG. 1, current i.sub.1 of the circuit at one side of the position 6 not including a.c. source 3 decreases below current i.sub.2 of the remainder of the circuit including a.c. source 3. In this respect, current transformers 4 and 5 are usually provided to measure and detect the difference between currents i.sub.1 and i.sub.2 so as to actuate a protective relay to open and protect the circuit against a dielectric breakdown at any point of the wire 2.
The current trouble detection system of FIG. 1 is considered suitable for use in a heating pipe system which is relatively short, namely up to several hundred meters. However, if the pipe system is as long as several kilometers, the detection system of FIG. 1 is unreliable and expensive because it requires an extremely long conductive wire for current difference detection by current transformers 4 and 5.
The present inventor attempted to overcome the drawback of the detection system of FIG. 1 and proposed an invention disclosed in Japanese Patent Office Gazzette No. 14018/1974 (Pat. No. 753,736) which is outlined hereinbelow, referring to FIG. 2.
A steel pipe which corresponds to pipe 1 of FIG. 1 is divided into two parts, one of which is relatively short pipe 15 to serve as a reference pipe and the other is a relatively long pipe 7 to form a main pipe. Between the two pipes are interposed detectors 10 and 10' for detecting the difference between currents i.sub.1 and i.sub.2 to find a current trouble in the heating pipe system when a dielectric breakdown 11 occurs in wire 8 extending along and inside pipe 7.
It should be noted here that current difference detector 10 or 10' is not accurate in detection of the current difference if the difference between currents i.sub.1 and i.sub.2 is less than 2%, thus causing unreliable protection of the system. More specifically, if the entire length of main pipe 7 is 1 and the length of an end portion 19 (from the dielectric breakdown position 11 to one end of main pipe 7 more remote from reference pipe 16) is x(0.ltoreq.x.ltoreq.1), current i.sub.1 in main wire 8 is substantially inversely proportional to the remainder of main pipe 7, namely 1-x. If length x is less than 2% of the entire length of main pipe 7, the decrease of the current, namely i.sub.2 -i.sub.1, due to the dielectric breakdown will be less than 2% of current i.sub.2. Thus, the system is not reliably protected because of a possible detection error of detector 10 or 10'.
From this point of view, the prior invention by the present inventor proposed to increase the thermal stability and the voltage resistance of the insulating layer of the wire corresponding to end portion 19 and of reference wire 16 in the attempt to eliminate dielectric breakdown in these ranges.
This proposal is certainly inexpensive as compared to the arrangement of FIG. 1 if it is applied to heat and keep warm a pipeline wherein reference pipe 15 will be several tens meters long, main pipe 7 is 10 kilometers long, for example, and hence end portion 19 to be increased in thermal stability and voltage resistance of the insulating layer thereof will be 200 meters long approximately. However, it is impossible to completely eliminate dielectric breakdown at these positions despite the increased thermal stability and voltage resistance of the insulating layer, and the prior invention still involves the same problem that the detector will fail to detect such a dielectric breakdown, even if rare, at the said positions.